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Is Quantum Mechanics Self-Interpreting?

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Is Quantum Mechanics Self-Interpreting? Is Quantum Mechanics Self-Interpreting? Andrea Oldofredi∗ April 26, 2019 Abstract Fuchs and Peres (2000) claimed that standard Quantum Mechanics needs no interpre- tation. In this essay, I show the flaws of the arguments presented in support to this thesis. Specifically, it will be claimed that the authors conflate QM with Quantum Bayesianism (QBism) - the most prominent subjective formulation of quantum theory; thus, they endorse a specific interpretation of the quantum formalism. Secondly, I will explain the main reasons for which QBism should not be considered a physical theory, being it concerned exclusively with agents’ beliefs and silent about the physics of the quantum regime. Consequently, the solutions to the quantum puzzles provided by this approach cannot be satisfactory from a physical perspective. In the third place, I evaluate Fuchs and Peres arguments contra the non-standard interpretations of QM, showing again the fragility of their claims. Finally, it will be stressed the importance of the interpretational work in the context of quantum theory. arXiv:1904.10988v1 [quant-ph] 24 Apr 2019 ∗ Contact Information: Université de Lausanne, Section de Philosophie, 1015 Lausanne, Switzerland. E-mail: [email protected] 1 Contents 1 Introduction 2 2 Quantum Mechanics without Interpretation 3 3 Quantum Mechanics or Something Else? 4 4 Evaluating the Arguments Against Non-Standard Interpretation of QM 7 5 Closing Remarks: The Importance of the Interpretational Work 8 1 Introduction What is the correct interpretation of Quantum Mechanics? This question not only is as old as QM itself, but also it still remains without a definite answer. Indeed, it is remarkable that nowadays there is no agreement about this issue among the experts working on the foundations of the theory, although a plethora of different proposals has been given. Looking at the history of quantum theory, in fact, it is possible to individuate several strategies to solve this conceptual riddle: many expanded the quantum formalism invoking hidden variables, or modified it introducing stochastic terms in the Schrödinger’s equation referring to spontaneous random collapses of the wave function (see e.g. Dürr et al. (2013), Bohm (1952), and Ghirardi et al. (1986), Bassi and Ghirardi (2003) respectively). Other believe that different results of quantum measurements are actualized in diverse, causally unrelated universes (Wallace (2012)), still others circumvent the problem changing the logic underneath quantum theory (French and Krause (2010), Dalla Chiara et al. (2004)), while some authors propose a return to a neo-Copenhagenist view (Landsman (2017), Ladyman and Ross (2013)), etc. Among this jungle of alternatives, nonetheless, there are eminent physicists affirming that standard quantum mechanics does not need an interpretation. Fuchs and Peres (2000) indeed argued for the internal consistency of an “interpretation without interpretation of QM”: nothing more than the bare formalism of quantum theory is needed, according to the authors, for “using the theory and understanding its nature”. Although this paper is not extremely recent, it is still worth discussing it for three notable reasons: (i) it contains many subtleties concerning the issues of the interpretation of quantum mechanics - in general, it aims to explain the reasons for which people should not pay attention to the business of interpreting QM, in particular it provides arguments against non-standard interpretations of the theory -, (ii) it will give us the opportunity to analyze a rather peculiar position about the meaning of the quantum formalism, and (iii) its main moral still represents the position of the majority of physicists working with QM. In the following sections I will discuss the arguments advanced by Fuchs and Peres to support their “no-interpretation view”. I aim to show that their proposal is strongly interpreted, since the authors surreptitiously conflate QM with Quantum Bayesianism (QBism) - the most prominent subjective formulation of quantum mechanics. In the second place, I will explain the main reasons for which QBism should not be considered a physical theory, being concerned exclusively with agents’ beliefs and silent about the ontology and physics of quantum objects dynamically evolving in spacetime. Rather, it must be taken as an epistemological framework able to weight the agents’ subjective probabilities for measurement outcomes (as correctly pointed out in Jaeger (2009), Chapter 3). Consequently, it will be argued that the solutions to the quantum puzzles advanced by the exponents of QBism can not be considered physically satisfactory. In 2 the third place, I will evaluate Fuchs and Peres’ arguments contra non-standard interpretations of quantum theory, emphasizing the fragility of their claims. Finally, it will be stressed the relevance of the interpretational work in the context of the foundations of quantum physics. 2 Quantum Mechanics without Interpretation At the outset of their essay, Fuchs and Peres openly claim that the attention given by physicists and philosophers to the interpretation of standard quantum mechanics may lead people to a wrong impression concerning its validity and consistency; in fact, they reassure the readers right away affirming that “[i]f quantum theory had been in crisis, experimenters would have informed us long ago!”. The main moral of their paper is unequivocal and can be summarized by saying that (i) standard quantum mechanics is consistent, being able to provide precise predictions for measurement outcomes - or more precisely, they claim it is worthless to worry about its interpretation to use it in actual experiments -, and (ii) its validity is well confirmed by empirical evidence. Thus, there is no urgency to investigate what is the correct interpretation of the quantum mechanical formalism: given the extraordinary empirical success of QM, its standard interpretation - or better, its bare formal structure - is sufficient to utilize the theory in practical situations and to understand “its nature”. Consequently, the authors state, quantum theory does not need a particular interpretation, it is just self-interpreting. For the sake of clarity, it is worth stressing that the argumentative strategy employed by Fuchs and Peres concentrates exclusively on the empirical adequacy of quantum theory, without any attention to its physical content, i.e. the story QM tells about what are the objects and processes determining the observed measured outcomes to which they repeatedly allude. In this precise regard, the authors state that to appreciate the physics of quantum theory, one has to reject every kind of philosophical demand for which QM should (somehow) provide a description of an external, mind-independent reality existing at the microphysical regime, or to give us an intimate knowledge of it. Consequently, one should not endorse non-standard interpretations of quantum mechanics, given that - in a way or another - these latter stress the importance of providing such a realistic description of the objects and processes taking place at the microscopic regime which are physically responsible for the observed measurements results. However, this desire of realism - inherited by a classical worldview which is difficult to overcome -, leads only to formally more complicated theories which do not improve the predictive power of quantum theory. The authors firmly claim that QM, on the contrary, does not describe reality, being an algorithm designed to calculate probabilities for macroscopic events, which are “consequences of our experimental interventions”. Therefore, to search for a realistic description of the quantum world is a pointless effort denoting a dogmatic and conservative view about physics. Furthermore, there is no logical necessity, they conclude, that a physical theory should provide a realistic world-view of a specific domain of physical phenomena. In more detail, the authors argue that the wave function ψ, the central mathematical ob- ject of QM, represents only a formal expression of agents’ degrees of belief, i.e. their available knowledge, about a certain experimental situation. Wave functions, therefore, do not refer to anything real in the world, “in particular”, they wrote, “no wave function exists before or after we conduct an experiment”. Consequently, QM is explicitly defined as a theory applica- ble exclusively to measurement situations, and not to physical phenomena independent from an experimental setting. Moreover, this view about the nature of ψ implies that the dynami- cal process described by the Schrödinger’s Equation (SE) provides solely the evolution of the probabilities in the agent’s mind. Hence, there is no motion of quantum objects in spacetime according to the authors. It is a straightforward logical consequence, then, that also the collapse of ψ in measurement situations is not a physical process, i.e. it does not happen to quantum 3 systems. QM, therefore, it is not anymore a mechanical theory in the usual physical sense - i.e. a theory which specifies its fundamental objects and how they dynamically behave and interact in spacetime -, but it becomes a branch of probability theory, concerning exclusively agents’ degree of belief - or human knowledge - in experimental situations. To conclude the section, it is possible to summarize and characterize Fuchs and Peres theses about quantum theory, i.e. their interpretation without interpretation of quantum mechanics, as follows: 1. QM does not describe an external physical
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